The instant invention is founded on a set of theoretical concepts of what saves energy in the operation of a relatively large size three phase induction motor, and the associated criteria are as follows:
1. The power supplied to the motor must be clean, sinusoidal power. When feeding a motor with a distorted wave voltage:
(a) the motor's iron losses are increased, because of the higher harmonics at a given r.m.s. voltage level;
(b) the eddy losses, as well as any non-I.sup.2 R losses in the motor winding conductors, such as skin effect, stray losses, circulating current losses are frequency dependent, and sharply increase at the presence of harmonics;
(c) the presence of harmonics precludes going high enough in the motor's flux density and consequently with a given motor with the r.m.s. feeding voltage, deprive any power source correction device of the ability to reach a favorable level;
(d) as it will be shown herein below, failure to reach a voltage high enough precludes reaching a low enough level of I.sup.2 R in the motor winding;
(e) it further precludes a low-enough level of I.sup.2 R in the feeding conductors;
(f) the harmonics increase the eddy, parasitic, stray, etc. losses in the feeding conductors;
(g) the harmonics increase the eddy, parasitic, stray, etc. losses in the conductor conduit or bus duct housing, particularly in a steel conduit;
(h) when feeding a motor with a voltage having a non-sinusoidal shape, the current will be non-sinusoidal, i.e. it will contain harmonics. These harmonics will be propagated forward to the motor, reducing significantly its efficiency; they will be also propagated back to the source;
(i) on their way back to the source, they contaminate the power lines, and this has a deleterious effect on critical loads connected to the affected power lines, i.e. loads like computers, cat-scans, x-ray equipment, etc.;
(j) on their way back to the source, they will make the disc of the watt-hour meter run faster, which means the watt-hour meter will indicate still higher readings than what the already substantially increased consumption due to harmonics would be.
2. The only way to provide clean, sinusoidal power is to insert exclusively linear-magnetic devices between the utility's clean sinusoidal voltage source and the motor(s) to be driven. Linear-magnetic devices work on the rectilinear portion of the hysteresis curve, never reaching the saturation knee. Today's utilities use without exception solid rotor synchronous generators to produce 3-phase electrically, and these generate and deliver a clean sine wave voltage curve from no-load to any load. When a linear magnetic device is inserted between such a source and the motor, then by laws of mathematics, a strictly sinusoidal voltage wave, free from any harmonics is delivered to the motor.
3. In today's large motors the ratio of load loss (I.sup.2 R+the eddy-plug loss) to the iron loss can be assigned a value of 6 at rated voltage, rated load and rated frequency.
4. Since motor manufacturers must count with normal, non-regulated feeding voltage levels which fluctuate, the commercial motor's flux density must be designed at least 10% below the limit of the hysteresis curve's linearity. Consequently, when inserting a device providing reliable regulated voltage between the utility lines and the motor(s), a feeding voltage increased up to 10% above the motor's rated voltage will provide the device designer with a method to decrease the losses in the motor, the losses in the feeding power lines, and and the losses in the conduits.
5. Energy savings obtained in the feeding lines, the cable conduit or bus duct enclosure, and billing savings in the way the watthour meter counts, are just as important as the energy savings obtained within the motor proper.
6. From what has preceded, it should be manifest that it is advantageous to feed the motor at full load with as high a voltage as flux density limitations permit. The flux density must still stay below the knee, preferably below the end of the hysteresis curve's linear portion, which can safely be assumed as being 10% over the rated excitation or being 10% applied over-voltage. When one increases the feeding voltage by this margin, the following will happen:
(A) Motor slip will decrease, consequently the speed of rotation of the motor will slightly increase. Assuming that the particular motor drives a compressor, through the slightly increased speed, the efficiency of the compressor will slightly improve. Consequently the HP demand on the motor's shaft will slightly decrease. This decrease constitutes the first saving in energy consumption.
(B) Even if (A) did not exist, with the higher feeding voltage applied, the motor load current (the in-phase components) decreases inversely proportional to the applied feeding voltage. With (A) existing, the decrease is still greater. With the current decreased, the I.sup.2 R motor winding losses would decrease proportionally to the square of the increase of the applied voltage, given constant temperature. Due to the subsequent lower current density in the winding conductors and due to the associated temperature drop, the resulting I.sup.2 R losses decrease in a higher power of inverse proportion compared to what the applied voltage increase would be.
(C) The motor winding eddy plus losses decrease at a much greater rate than what the squared increase of the applied voltage would be. As before under the term "eddy plus", all non-I.sup.2 R losses are meant, including stray, parasitic, circulating current and skin effect losses. They are considerable in large motors. These losses are obtained from the wattmeter readings minus the I.sup.2 R losses, as determined by a DC resistance premeasurement corrected to the actual reference temperature and multiplied by I.sup.2.
(D) The iron losses increase with an increase of the applied voltage. From statistical analysis of test data, with a 10% overexcitation, in the worst case the iron losses will double using today's electrical steels. Yet, since as mentioned, in today's large motors at rated excitation, the iron loss amounts to no more than 1/6 of the load loss at rated current, thus doubling such a loss, is many times outweighted by the reduction of the winding losses.
(E) As in previously discussed considerations, through such an increase of the voltage applied to the motor, energy savings are not limited to the motor alone. They go further to the feeding conductors and feeding structures. Due to the increase of the motor feeding voltage, the I.sup.2 R losses in the feeding cables and/or busses decrease more than in proportion to the square of the increase of the voltage, the excess above the square ratio being again caused by (A), and the lower cable and/or bus temperature.
(F) For reasons explained with regard to the motor, in cables/busses the eddy losses decrease in a substantially larger proportion than the I.sup.2 R losses.
7. For reasons of safety, a large motor which works over-excited by as much as 10%, should not be turned on directly across the line. The device inserted between the utility and the motor(s) must prevent such a turn-on, and automatically reset the control circuitry in such a way as to confine every start to the device's position of lowest available motor feeding voltage, thereby also providing a soft start. Applicant's patent application Ser. No. 271,202 filed on June 9, 1981, and since matured into U.S. Pat. No. 4,438,387, covers this function and is incorporated in its entirety into this application by reference.
8. As it is known, unbalanced phase voltages cause additional motor winding load losses. Consequently, the device must include power and control means to accurately equalize the phase voltages fed to the motor, to prevent the occurence of these additional losses in cases of unbalanced incoming line voltages and/or unbalanced loads. Unbalanced utility voltages within .+-.5% are rather the rule than the exception.
9. When a motor runs idle, it delivers no useful work, yet it consumes a significant amount of reactive power, which has an immediate effect seen in the magnitude of the magnetizing current. The magnetizing or exciting current produces I.sup.2 R and other losses in the feeding cables/busses, which add to motor's no-load losses.
One remedy is to turn off the motor(s) for a length of the periods in which they perform no useful work. The other way is to reduce the feeding voltage and compensate the reactive power.
It has been mentioned in 6 (D) that overexciting by 10% will cause (in the present electrical steels) the no load losses to double. Likewise, reducing the motor's feeding voltage by 105, will reduce both the no-load losses and the motor's magnetizing current to approximately one half of their values at the rated voltage. If, as mentioned, the no load loss at the rated voltage amounts to 1/6 of the load loss at the rated voltage plus rated load, then by reducing the voltage by 10% at idle run, will bring the no load loss down to 1/12 of the load loss at the rated voltage and rated load. For all practical purposes such a loss is optimum; there is no benefit achieved by going with the feeding voltage still lower than to those 10% below the rated voltage. Therefore, a device whose task is energy saving for motors must embody the capability of not only raising the voltage up to 10% above the rated voltage magnitude, but also in the direction below the rated voltage, never more than maximally 10%, with as little as 3% being sufficient in many cases.
Criteria 1 and 2 are important; in order to save energy, a pure sine wave power supply is required. With solid state devices (which cannot but distort the wave), there is always energy waste, and never energy preservation. Criteria 4-6 demonstrate how substantial energy savings can be achieved by means of regulating overvoltage feeding, which refers to motors when they deliver a load from 50% rated load to overload. The method also provides means for "stretching" the HP capacity of a motor, which is an added substantial benefit provided by this invention. The energy saving, which can exceed 30%, should be of utmost importance not only for the individual motor load consumer, but there is a great importance for it nationwide and worldwide. Criterion 7 describes a necessary safety measure. Criterion 8 demonstrates that unbalanced phase voltages cause substantial additional energy waste. By this invention such waste is also eliminated. Criterion 9 describes energy waste at no load and low load motor run which at times may be significant, but never as important as the losses at substantial loads. This invention provides a solution therefor.
Prior art, U.S. Pat. No. 4,052,648 granted to Frank Nola, of NASA, aims at energy savings for fractional HP motors running idle and on low loads. Even under these limited conditions, savings of energy consumed by the motors and feeding lines are problematic, because the motors are fed from the Nola device, which provides voltage of a deformed shape with respect to a sine wave. At any given HP load, including idle run, watt losses are substantially higher for a deformed voltage wave. To these increased motor losses, losses of the thyristor drive of the Nola device are added, and then the losses are also increased in the feeding lines; these loss increases are being compared to the losses with the same motor(s) at the same motor load, at the same rms feeding voltage, but with sine wave power fed. If Nola provides advantages compared with a non-regulating motor drive, these advantages are limited to fractional HP motors, running idle most of the time. From an energy standpoint, the large size motor and the motor subjected to continuous loads present a substantially more important national and worldwide energy preservation problem than all fractional horsepower motors.